1. Field of Invention
The present invention relates to a toner used for the development of an electrostatic image formed by an electrophotographic method or an electrostatic recording method using a developer. The present invention also relates to a method of preparing the toner, a developer for electrostatic charge image using the toner, and an image formation method using the developer for electrostatic charge image.
2. Description of Related Art
Methods for visualizing image information via an electrostatic charge image, such as an electrophotographic method, are currently used in various fields. In the electrophotographic method, an electrostatic charge image is formed on a photosensitive element via a charging and an exposure process; the image is developed with a developer containing toner; and the image is visualized via a transfer process and a fixing process.
As the developer, there are two-component-type developers containing toner and carrier and one-component-type developers containing only magnetic toner or non-magnetic toner. Toner particles in both of these types of developers are generally prepared by a kneading and pulverizing method, in which a thermoplastic resin is melted and kneaded with a pigment, a charge controller and a release agent such as a wax; cooled; finely pulverized; and classified. In order to improve the flowability and cleaning property of the toner particles prepared by the kneading and pulverizing method, inorganic or organic fine particles are added to the surface of the toner particles as needed.
Toner particles prepared by known kneading and pulverizing methods generally have an amorphous shape without a homogeneous surface composition. Although the shape and surface composition of toner particles change slightly depending on the pulverizability of the materials used and conditions of the pulverizing process, it is difficult to intentionally control these elements. In addition, when toner particles are prepared using a material with a particularly high pulverizability, the particles are often pulverized more finely in the developing device due to mechanical forces such as shearing force and the shape of the toner particles is thereby altered.
As a consequence, in the two-component-type developer, the pulverized toner particles adhere to the carrier surface so that the charge deterioration of the developer is accelerated. In addition, in the one-component-type developer, the particle size distribution of toner particles is expanded such that the pulverized toner particles are scattered or the developing property is lowered based on the change in toner shape, resulting in a deteriorated image quality.
In addition, when the toner particles have an amorphous shape, even if an auxiliary agent for improving flowability is added, the flowability may be insufficient. In addition, the fine particles of the auxiliary agent for improving flowability may move to the concave portions of the amorphous toner particles and be buried therein during operation due to mechanical forces such as the shearing force. As a result, flowability of the toner decreases over time and/or the developing property, transfer property, and cleaning property are deteriorated. Furthermore, if such toner is recollected by cleaning to return to the developer, the image quality tends to be further deteriorated. In order to prevent these problems, increasing the amount of the auxiliary agent for improving flowability has been considered. However, this may involve problems in that spots on the photosensitive element may be generated and the auxiliary agent for improving flowability may be scattered.
On the other hand, in a case of a toner containing a release agent such as wax, the release agent may be exposed on the toner particle surface depending upon the combination with a thermoplastic resin. Particularly in the case of a toner combining a resin that has a high-molecular-weight component that provides the resin with elasticity and that is not easily pulverized and a vulnerable wax such as polyethylene, exposures of the vulnerable wax such as polyethylene on the toner particle surface is often observed. Although such a toner has an advantage in the releasing property at fixing or in cleaning of untransferred toner on the photosensitive element, reliability as a developer is decreased since polyethylene on the surface of the toner particles easily falls off the toner particles due to forces in the developing device such as the shearing force and transfers to the developing roll, the photosensitive element, and the carrier, etc., causing dirt.
Under such circumstances, in recent years, as a means for preparing a toner whose particle shape and surface composition are intentionally controlled, toners have been prepared extensively with a wet process. As the wet process, there are several methods that are often used, namely, a wet sphering method capable of controlling shape of toner particles, a suspension granulating method capable of controlling surface composition of toner particles, a suspension polymerization method capable of controlling even the internal composition of toner particles, and an emulsion polymerization agglomeration method. However, other wet methods have also been used.
In recent years, one of the bigger problems associated with having toner prepared by the wet process is adding a so-called surfactant or dispersing agent (which is also referred to as "stabilizing agent" hereinafter) to greater or less degrees so as to control or keep the toner particle diameter. If the above-mentioned stabilizing agent is added at the time of toner particle production, the stabilizing agent remains in the toner liquid at the time the reaction is completed. A large amount of the stabilizing agent even remains on the surface of the toner particles.
If the stabilizing agent remains in the toner, it may lower the charging and the resistance of the toner. In particular, the stabilizing agent may have a negative influence at high temperature and humidity. Thus stable developing and transfer properties of toner may not be attained. Therefore, the advantages of the wet process, in that the particle diameter distribution of the toner particles can be made sharp and a highly precise image can be realized by making toner particles having small particle size, may be damaged. In addition, pollution on the surface of the toner leads to decreases in the flowability and preservation property and to decreases in reliability. Thus, a cleaning process to remove the stabilizing agent from toner particles is conducted, particularly after toner particles are formed by a wet process.
Most of the known methods for removing stabilizing agents from toner particles involve washing the toner particles with water. However, it is impossible to completely remove the stabilizing agent adhered to the surface of toner particles by these methods. In addition, a large amount of stabilizing agent floating in a solution cannot be easily separated. In addition, the amount of water required for cleaning is huge if the stabilizing agent is decreased as much as possible. Further, such as with toner prepared by an emulsion polymerization agglomeration method, the stabilizing agent that remains inside the toner theoretically cannot be removed.